Revert "[SCEV] Don't invalidate past dependency-breaking instructions"

Unforuntately, the assumption underlying this optimization is
incorrect for getSCEVAtScope(): A SCEVUnknown instruction with
operands that have constant loop exit values can evaluate to
a constant, thus creating a dependency from an "always unknown"
instruction.

Losing this optimization is quite unfortunate, but it doesn't
seem like there is any simple workaround for this.

Fixes #68260.

This reverts commit 3ddd1ffb.